This application claims the priority of German patent document 198 53 126.5, filed in Germany on Nov. 18, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for controlling the ride height of a wheeled vehicle, and to a wheeled vehicle with a ride-height control system.
German patent document DE 195 46 728 A1 discloses a ride-height control device for a motor vehicle having a body supported on the vehicle axles by a pneumatic support in the form of an air suspension bellows. Wheels with gas-filled tires are provided on the vehicle axles. The height of the vehicle body relative to the vehicle axles can be changed by changing the quantity of air in the air suspension bellows. A plurality of height sensors continuously determine the distances between the vehicle body and the axles, so that this distance can be adjusted to a predefinable setpoint value. In addition, pressure sensors are provided for sensing the pressure in the air suspension bellows, and the sensed pressure value (which is directly dependent on the payload of the motor vehicle) is used to determine an auxiliary variable which is indicative of deformation of the tires. The ride-height control compensates for such deformation of the tires caused by the load, by appropriately taking into account the auxiliary variable in the adjustment of the setpoint value.
German patent document DE 35 12 047 A1 discloses a wheeled vehicle with front and rear wheel suspension units, on which a vehicle body is supported by a vertically adjustable hydraulic or pneumatic supporting device.
A variable pressure can be applied to the two supporting devices via a common controller and a common pressure supply, to adjust the distance between the vehicle and the wheel suspension units. The vertically adjustable supporting device is controlled such that the change in the height of the vehicle takes place as a function of the travel velocity, specifically in such a way that it is set higher at a low travel velocity and lower at a high travel velocity.
German patent document DE 41 12 738 A1 discloses a system for sensing fixed, type-related data of a motor-vehicle tire by means of a sensing unit on a motor-vehicle body. The contactless sensing device scans a data carrier which is permanently connected to the tire, and which contains, for example, information on the material, the geometry and/or other properties of the tire. Preferably it is made available by the tire manufacture.
German patent document DE 33 15 594 A1 discloses a device for monitoring the position of a motor-vehicle body relative to an underlying surface, in which a first distance-measuring device determines the distance between a wheel suspension unit and the underlying surface, and a second distance-measuring device determines the distance between the vehicle body and the underlying surface. The distance-measuring devices are in the form of transceiver units for measuring the propagation time of electromagnetic waves, and are arranged spaced apart from one another on the bodywork of the vehicle, thus permitting the angular position of the bodywork relative to the underlying surface to be determined. Distance-measuring devices, assigned to individual wheels, on the wheel suspension units permit the air pressure of the corresponding wheel tire to be determined. An evaluation circuit determines the position of the bodywork and actuates an actuating element which adjusts the angular position of the vehicle headlight.
German patent document DE 44 43 810 A1 discloses a device for controlling the ride height of a motor-vehicle body relative to an underlying surface, which device measures in a contactless fashion the distance between the vehicle body and underlying surface, using a sensor which is mounted on the vehicle body, by measuring the propagation time of an electromagnetic signal. The signals of the sensor are fed to an electronic evaluation unit which drives actuators which interact with the vehicle suspension system and regulate the ride height of the vehicle.
One object of the invention is to provide a method and apparatus of the type mentioned above, for controlling the ride height on a wheeled vehicle, which uses the simplest possible means to sense and position the vehicle precisely with respect to an underlying surface.
This object is achieved by the method and apparatus according to the invention for controlling the distance between a vehicle body and a wheel suspension unit, in which the distance setpoint value is set as a function of both the vehicle payload (i.e., the wheel load) and the tire pressure. These two variables are used to determine the geometry of the tire in terms of the height of the wheel axle above the underlying surface (i.e., in particular the radius of the tire between the wheel axle and underlying surface) expediently using tire-specific characteristic data. Information concerning the tire geometry is then included as a correction value in the calculation of the distance setpoint value.
In one embodiment of the invention, a wheeled vehicle has a tire-pressure sensor (i.e., a gas pressure sensor), whose signal is supplied together with a signal of a sensor for sensing the vehicle payload (in particular the wheel load), to a regulator unit which determines the axle height between the wheel suspension unit and the road, in order to correct the distance setpoint value between the vehicle body and wheel suspension unit.
In another embodiment of the invention, the distance setpoint value is determined as the difference between a preset constant distance between the vehicle body and an underlying surface, and an axle-height correction value. The axle-height correction value, which is determined with reference to predefined characteristic curves as a function of the vehicle payload and the tire pressure, describes the geometry of the tire at a given time by representing the distance between the wheel axle and the underlying surface. It is not influenced by the method for controlling ride height; instead, when the axle-height correction value changes, the distance which is to be set between the vehicle body and the wheel suspension unit is changed (in order to maintain the distance between the vehicle body and the underlying surface constant even when the tire geometry and thus the axle-height correction value changes). Here, the predefined characteristic curves describe the axle height (also referred to as the so-called static tire radius) as a function of the wheel load and the tire pressure.
According to a further feature of the invention, the axle-height correction value is determined as the sum of at least a tire-pressure-specific correction component and a payload-specific correction component, the latter being determined from the measured load of a spring element which supports the vehicle body against the wheel suspension unit. The tire-pressure-specific correction component and the payload-specific correction component can each be calculated separately from one another and easily added in order to obtain the axle-height correction value. In this regard, the load of a spring element which is in the form of a hydraulic suspension unit or pneumatic suspension unit can be sensed particularly easily as a pressure within the spring element or within the assigned hydraulic or pneumatic system.
In a further embodiment of the invention, the distance setpoint value also varies as a function of a sensed travel velocity or wheel rotational speed. As a result, the tire geometry which changes with the travel velocity of the vehicle (or the rotational speed of the wheel) is taken into account by means of an appropriate correction of the distance setpoint value. Preferably, the corresponding dynamic tire radius is determined for a specific velocity with reference to a predefined, tire-dependent characteristic curve and is taken into account, as velocity-dependent correction component, in the determination of the axle-height correction value.
In a further refinement of the invention, a wheeled vehicle is provided with a vehicle body supported by a plurality of hydraulic or pneumatic spring elements, on respective wheel suspension units, each having a wheel with a gas-filled tire mounted thereon. The ride-height control system has a central hydraulic or pneumatic pressure supply for the spring elements with a pressure sensor, a compressor unit and a valve block which can be actuated by a central regulator unit. It is thus possible to couple the spring elements individually to the compressor unit and the pressure sensor by means of the valve block. In such an arrangement, only a central pressure sensor and a central compressor unit are necessary to supply pressure and monitor the pressure for a plurality of spring elements. The spring elements can successively be coupled to the compressor unit and the pressure sensor by means of the valve block, so that a particularly simple design of the ride-height control system is obtained.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.